Edward II was born in Caernarfon on Tuesday 25 April 1284, St Mark's Day, and sixteen days after Easter Sunday. His father Edward I was then almost forty-five (born 17 June 1239) and had been king of England for just under eleven and a half years, since the death of his father Henry III on 16 November 1272. Of Edward I's three siblings who had survived childhood, only one was still alive in 1284: Edmund, earl of Lancaster, Leicester and Derby, who was married to Blanche of Artois, niece of Louis IX of France and dowager queen of Navarre. The couple had two sons, Edward of Caernarfon's first cousins Thomas and Henry, born in about 1278 and 1281 respectively. Edmund lived until 5 June 1296, and Blanche until 2 May 1302.
Edward of Caernarfon's mother Queen Eleanor (d. 28 November 1290), born Infanta Doña Leonor de Castilla and known as Alianore in England, was probably forty-two at the time of her youngest child's birth (and it is virtually certain that Edward was indeed her youngest child; his alleged younger sisters Beatrice and Blanche, who appear in two of Alison Weir's books, are inventions of much later centuries). Eleanor's brother Alfonso X of Castile died exactly three weeks before Eleanor gave birth to Edward, so presumably she spent the end of her pregnancy in mourning for him. He was succeeded as king by his second son Sancho IV, who ignored the claims of his young nephews, the two sons of his dead older brother Fernando de la Cerda. Of Queen Eleanor's fourteen siblings - who included the archbishops of Seville and Toledo - only two were still alive on 25 April 1284: her sister Berenguela, a nun at the abbey of Las Huelgas near Burgos, northern Spain, and the colourful Don Enrique, lord of Écija, Medellín, Dueñas and many others, senator of Rome, mercenary in North Africa and (later) regent of Castile for his great-nephew Fernando IV, who in 1284 was sixteen years into a thirty-year imprisonment in Naples.
Edward of Caernarfon was not born as heir to the English throne, as his ten-year-old brother Alfonso of Bayonne, named after their uncle and his godfather Alfonso X, was then still alive. Alfonso died suddenly on 19 August 1284, thus sadly depriving England of its King Alfonso, and the four-month-old Edward then did become their father's heir, the sole survivor of Edward I and Eleanor of Castile's four sons (John and Henry having already died in 1271 and 1274). Three days before young Alfonso's sudden death, Edward of Caernarfon's second cousin and future father-in-law got married, probably in Paris. Sixteen-year-old Philip, eldest surviving son of Philip III of France whom he succeeded as king the following year, married Joan (or Jeanne or Juana), probably then eleven years old and queen of Navarre and countess of Champagne and Brie in her own right. Joan was the only surviving child of Edward of Caernarfon's aunt by marriage Blanche of Artois, above, and her first husband King Enrique I of Navarre. Philip and Joan's marriage produced four children who lived into adulthood: Louis X, Philip V, Charles IV and Edward II's queen Isabella of France.
Five of Edward of Caernarfon's numerous (at least ten) older sisters were still alive at the time of his birth, and also survived into adulthood: Eleanor (born 1269), Joan (born 1272), Margaret (born 1275), Mary (born 1279) and Elizabeth (born 1282). The only one of his grandparents alive in 1284 was his paternal grandmother Eleanor of Provence, dowager queen of England and widow of Henry III, who was probably in her early sixties at the time and lived until June 1291. Edward's paternal aunts Margaret and Beatrice had been dead for nine years, but their husbands were still alive. Alexander III, king of Scotland, widower of Margaret, was in his early forties, and died in an accident on 19 March 1286 when he rode his horse off a cliff during a fierce storm, leaving as his sole heir his little granddaughter Margaret of Norway, to whom Edward of Caernarfon was betrothed in 1289. And finally, Duke John II of Brittany, widower of Edward I's sister Beatrice, lived to the ripe old age of sixty-six and died in a bizarre accident in November 1305: a wall fell on him and crushed him to death as he led the horse of the newly elected pope, Clement V, around Avignon.